Indoor spaces are commonly ventilated by means of the controlled introduction of outdoor air. When outdoor air must be conditioned prior to its introduction into interior spaces to meet human comfort or industrial standards, the amount of energy required for ventilation increases sharply. To reduce energy usage, air-to-air heat exchangers are frequently employed to recover energy from building exhaust air. In winter, heat is transferred from the warm exhaust air to the cold incoming air. In summer, heat is transferred from the incoming fresh air to the relatively cooler exhaust drawn from air-conditioned spaces. Air to air heat exchangers are also applied to transfer energy in recirculation of building air, drying air or gas processes and other heat transfer processes. Heat exchangers that operate in this way are commonly built from an assemblage of parallel plates of aluminum or plastic, and hence are referred to as parallel-plate heat exchangers (see U.S. Pat. Nos. 4,051,898; 4,874,042; 5,033,537; 4,006,776; 4,858,685). When the two air streams pass in opposite directions, the exchanger is said to be counter-flow. More commonly, the two air streams pass at right angles to one another in adjacent flow passages that are separated by the plates, and the exchanger is said to be crossflow. The plates allow a high degree of thermal contact between the two air streams, but prevent direct mixing.
It is advantageous, but considerably more difficult, to transfer water vapor between the streams in addition to heat, especially when the transfer of gaseous pollutants between the air streams is unacceptable. In winter, valuable humidity can be recovered from the exhaust and transferred to the dry, fresh air. In summer, the relative humidity of the incoming fresh incoming air can be reduced if moisture is transferred from it to the relatively drier exhaust air.
Devices are currently available for transferring both heat and water vapor. They are classified either as regenerators or as porous-plate, air-to-air recouperators. Regenerators are designed to enable two different air streams to pass successively over a single heat and mass transfer medium. The medium is often a rotating disk, known as a heat wheel, which continuously rotates through two side-by-side but separate air streams. Heat and moisture are absorbed by the rotating medium as it passes through the hotter or more humid air stream and are rejected from that same medium as the heat wheel rotates into the cooler or less humid air stream (see U.S. Pat. Nos. 3,065,956; 3,398,510; 5,869,272; and 5,183,098). Heat wheels incorporate many moving parts that are prone to wear and failure. Furthermore, air leakage between the streams is inevitable. Pollutant gases entrained in one stream leak through the seals separating the air streams as the medium rotates. The amount of pollutant leakage depends on the construction of the mechanical air seals and upon the magnitude and direction of the pressure difference between the air streams.
Porous-plate, air-to-air recouperators are intended to transfer both heat and water vapor. They contain no moving parts, and are comprised of a series of parallel plates. The porous plates used in currently available recouperators are made from treated paper rather than aluminum or plastic (see U.S. Pat. Nos. 2,478,617; 2,986,379; 3,166,122; 3,666,007; 4,550,773; and 4,051,898; and Japanese Pat. Doc. 60-205193). In general, current porousplate recouperators leak air and pollutants between the air streams in addition to the moisture transfer.
U.S. Pat. No. 4,051,898 (Yoshino, et al.) describes a porous-plate exchanger made from paper treated with a moisture-absorbing compound such as polyvinyl alcohol. A commercially available, Yoshino-patented exchanger was evaluated by Fisk, et al. (Fisk, W. J., B. S. Pedersen, D. Hekmat, R. E. Chant, H. Kaboli, "Formaldehyde and Tracer Gas Transfer between Airstreams in Enthalpy-Type Air-to-Air Heat Exchangers", ASHRAE Transactions, 91, 173 (1985)) who determined that the Yoshino exchanger possessed an effectiveness for water vapor transfer of 28%. The effectiveness, E, compares the actual transfer to the maximum possible transfer between the streams under ideal conditions. An effectiveness for water vapor transfer of at least 25% is preferred.
Fisk, et al. (1985) also measured the effectiveness of the same porous-plate exchanger for the transfer of three representative pollutant gases. The measured values were 10.3% for formaldehyde, 7.3% for propane, and 6% for sulfur hexaflouride. These rates of pollutant transfer are considered too high for air quality sensitive applications, and pollutant transfer concerns have hindered the acceptance of porous-plate exchangers.
Another version of the porous-plate exchanger, described in Japanese Pat. Doc. 60-205193 by Takahashi, et al., incorporated a microporous polymer film saturated or coated with a moisture-absorbing substance. That substance was a combination of a hydrophilic polymer, such as polyvinyl alcohol, and a hygroscopic, inorganic salt, such as lithium chloride. Liquid water, appearing within the pores as a result of the hydration of the inorganic salt, served to plug the pores, preventing air transfer through the film. A range of pore diameter was chosen, 0.1 to 10 microns, which allowed liquid-phase mass transfer within the pores but which prevented blow-out of the liquid by the air pressure differential imposed by the ventilation system. The coated-film exchanger had a mean effectiveness for water vapor transfer of 63%. Its effectiveness for carbon dioxide transfer, measured at 3%, is still too high whenever air quality is a priority.
For the purpose of comparing mass exchanger devices, the selectivity, S, of an exchanger for water vapor with respect to a pollutant gas may be calculated from a ratio of the effectiveness. EQU S=.epsilon.(water vapor)/.epsilon.(pollutant) Equation 1
From the results of Fisk, et al. (1985), the selectivity of the paper exchanger for water vapor relative to formaldehyde was 2.7. For water vapor relative to propane the selectivity was 3.8, and for water vapor relative to sulfur hexaflouride, 4.7. The coated-film exchanger of Takahashi was more selective, providing S=21 for water relative to carbon dioxide.
In addition to their low-to-moderate selectivities, current models of the porous-plate exchanger have other drawbacks. In terms of mechanical properties, the structural integrity of the treated paper exchanger is compromised when the paper becomes wet as a result of condensation. Condensation is likely to occur in porous-plate exchangers during cold weather operation. The paper absorbs moisture, swells, and weakens the exchanger structure. When air at temperatures below 32.degree. F. is present, the moisture-laden paper freezes and cracks. The air-separating structure is broken and is incapable of preventing excessive gaseous pollutant leakage.
Coated films incorporating water-soluble compounds like the barrier proposed by Takahashi for use in air to air heat -moisture exchangers is impractical. Under cold-weather, condensing conditions, any liquid water condensate contacting the salt-bearing liquid within the pores leaches the salt from the film. With repeated exposure to condensate, the accessible salt is stripped away, leaving behind air-filled pores which are less effective at blocking the transfer of pollutant gases.
Thin-film composite membranes have been used in the fields of reverse osmosis and gas separations but not heretofore for air-to-air heat and moisture exchangers. Reverse osmosis and gas separation applications are characterized by very large cross-membrane pressure differences (up to 1000 psi) and by relatively low rates of fluid flow, while an air-to-air heat and moisture exchanger application is typified by cross-stream pressure differences of 1 inch of water (0.036 psi) and high rates of fluid flow (hundreds of cubic feet per minute).
A critical process in the development of thin-film composite membranes is interfacial polymerization (Cadotte et al., U.S. Pat. No. 4,259,183 and 4,277,344). The disclosures of Cadotte et al. are incorporated herein by reference. During interfacial polymerization, a porous supporting material is first saturated with a monomer-bearing solution. A second solution, immiscible with the first, is then contacted with one or both surfaces of the porous support. The second solution contains a monomer which reacts rapidly with the first monomer to produce, via a condensation reaction, a polymer that is often covalently bonded to the porous support. The formation of the polymer film at the interface of the two solutions separates the reagents, limiting the forward progress of the reaction. The self-limiting nature of the reaction results in a non-porous film that is very thin and yet continuous (see Cadotte, et al., "Thin-Film Composite Reverse-Osmosis Membranes: Origin, Development, and Recent Advances," in Vol. I of Synthetic Membranes, ACS Symposium Series 153 (1981)).
The application of interfacially polymerized membranes to gas separations is exemplified by the oxygen/nitrogen system (U.S. Pat. No. 4,493,714) and the oxygen/nitrogen/carbon-dioxide/hydrogen system (U.S. Pat. No. 4,963,165). For the drying of compressed air, a thin-film composite membrane in the form of a hollow fiber was developed. A bundle of such hollow fibers was fed with a high pressure mixture of the gases to be separated [Wang et al., "Hollow Fiber Air Drying," J. of Membrane ci., v. 72, pp.231-244, 1992]. Many interfacially polymerized membranes developed for reverse osmosis applications incorporate condensation polymers which are highly hydrophilic and potentially suitable for separations involving water vapor (U.S. Pat. Nos. 4,876,009, 5,593,588, 4,259,183, and 4,277,344). All of these applications are at high cross-membrane pressure drops and/or at low rates of fluid flow.